It's likely the same issue with GSX + FSLTL: together they might reach the maximum number of SimObjects the sim can display, which will cause objects to start disappearing at random.
Nobody can really fix this, other than MS/Asobo. In the meantime, what you can do from GSX is:
- Disable "Ground clutter", this will help ( at *default* airports only ) with keeping the overall number of objects under control, since GSX won't add extra objects on jetway replacement airports, but it will just replace 1 default jetway with 1 custom jetway.
- Lower the passenger Density. The higher the setting, the more objects you'll have in the sim all together at any given time.
From the AI traffic product, you can:
- Lower the overall Traffic density
- Disable ground services for AI planes ( FS Traffic has a configurator option to do this )